1. Field of the Invention
The present invention relates to a method of verifying the integrity of an encryption key obtained by combining key portions and used in association with a symmetrical encryption algorithm.
2. Brief Description of the Related Art
It is known that an encryption algorithm operates by means of a key, which constitutes an element that is essential for ensuring that the encryption is secure.
Various means are known for attempting fraudulently to obtain the key associated with an encryption algorithm. One such means comprises analyzing electrical or electromagnetic phenomena that occur when the key is transferred from storage in a read-only memory to a read/write memory or from the read/write memory to a register. In order to combat that method of attack, it is known to subdivide the key into a plurality of portions, generally two portions, that are combined by means of an operator, e.g. the commutative EXCLUSIVE OR operator, in a read-protected zone in which the algorithm is implemented.
Another means for reconstituting the key consists in provoking disturbances in the key and in analyzing the consequences on the encryption of a data item that is used repetitively which provoking successive disturbances to the encryption key. To prevent an attacker from performing successive disturbances to the key, it would be desirable to be able to verify the integrity of the key when implementing the encryption algorithm. Unfortunately, combining key portions in a protected zone that is not accessible for reading prevents any reading back of the key in order to verify its integrity.